Recovery of zinc values from zinc plant residue

ABSTRACT

In a process of recovering metal values from leaching residues of zinc-bearing materials such as the leaching residues of roasted zinc concentrate, comprising subjecting said leaching residues to a subsequent leaching with a solution of dilute sulphuric acid for dissolving the zinc, adding a sulphur-bearing ore to said solution, separating a sulphurous residue from said solution, oxidizing said solution and precipitating the therein contained iron by using an oxidizing agent and adding a neutralizing agent containing zinc oxide.

United States Patent Bodson [4 Mar. 28, 1972 [s41 RECOVERY OF ZINCVALUES FROM E61 Reierences Cited ZINC PLANT RESIDUE UNITED STATESPATENTS [72] 59"; {*F Angku" 1,973,300 9/1934 Thompson ..23/12s 6 8mm3,193,382 7/1965 Veltman et al. [73] Assignee: Soeiete des Mines etFonderies de Zinc de ,434,947 3/1969 Steintveit La Vieille Montague,S.A., Angleur-Liege, 3,493,365 2/ 1970 Pickering et al ..75/l0l BelgiumI Primary Examiner-L. Dewayne Rutledge [22] filed 1969 AssistantExaminer-G. T. Ozaki [21] Appl. No.: 790,835 AttorneyYoung&Thompson 30Foreign Application Priority Data [57] ABSTRACT Nov 20 1968 Be] ium D 41993 In a process of recovering metal values from leaching residues g ofzinc-bearing materials such as the leaching residues of [52] Cl l 75/10175/108 75/115 roasted zinc concentrate, comprising subjecting saidleaching i 75/120 residues to a subsequent leaching with a solution ofdilute s 1 1 Int. Cl. ..C22b 19/22 sulphuric acid dissdving Zinc adding58] Field of Search 75/120, 101 108 1 23/12 ore to said solution,separating a sulphurous residue from said 204/119 solution, oxidizingsaid solution and precipitating the therein contained iron by using anoxidizing agent and adding a neutralizing agent containing zinc oxide.

8 Claims, No Drawings RECOVERY OF ZINC VALUES FROM ZINC PLANT RESIDUEThis invention relates to a process of leaching residues of leachedzinc-bearing materials with the object of recovering certain elements,principally metals and in particular zinc.

In the electrolytic process in zinc metallurgy zinc-bearing materialssuch as roasted blendes are subjected to leaching in dilute sulphuricacid originating from electrolysis cells. In the course of thisoperation a quite considerable part of the zinc is not dissolved becauseit is fixed in certain compounds which are insoluble under theconditions of leaching, such as, in particular, zinc sulphide and zincferrites (ZnO'Fe O This insoluble zinc, together with other elements,particularly lead and silver, is found after settling and/or filteringin a residue, commonly termed the leaching residue.

The leaching residue contains a good many elements: some are insolublein sulphuric acid, among these CaO, Si and A1 0 being considered asnon-recoverable while Pb, Ag, Sn and Au are recoverable. Certain otherswhich are supposed to be soluble in sulphuric acid are neverthelessfound in the residue because of the conditions of leaching; among theseZn, Cu and Cd are recoverable, Fe, Mg, Cl, F, Na and K are considered asnon-recoverable and As, Sb, Ge, Co and Ni are deleterious as far assubsequent operations are concerned.

Various processes are already known making it possible to bring theabove-mentioned soluble rand recoverable elements into solution, toseparate the recoverable insoluble elements and to eliminate theinsoluble, and even dangerous, elements.

The problem forming the basis of this invention is to supply a processfor recovery, starting from leaching residues of zinc and other elementswhether or not they are recoverable.

This problem is solved by the present invention which relates to aprocess in which the leaching residue is subjected to a subsequentleaching at atmospheric pressure and at a temperature between 50 C. andboiling point, using a dilute sulphuric acid solution, whose initialfree acid concentration is between 100 grammes per liter and 250 grammesper liter, equal in quantity of two to five times the stoichiometricquantity of acid necessary to dissolve the zinc contained in the saidleaching residue, which forms a sludge hereinafter called the leadresidue, containing recoverable insoluble elements such as lead, silverand gold, suspended in a solution to which is then added, at atemperature between 50 C. and boiling point, a sulphur-bearing materialcontaining, e.g., zinc sulphide, a quantity of L1 to 1.5 time thequantity stoichiometrically necessary for the reduction to the ferrousstate of the ferric iron present in the said solution, a sulphurousresidue is separated from the said solution and the iron contained inthe filtrate is precipitated, utilizing as an oxidizing agent preferablythe oxygen of the air at a temperature between 50 C. and boiling pointand adding a neutralizing agent containing zinc oxide such as roastedzinc concentrate in the quantity necessary to maintain a pH value of 1.7to 3, so that the precipitating iron collects with it other impuritieslike arsenic, antimony, germanium and fluorine.

In the process in accordance with the invention, the treatment of thesolution before the stage of iron precipitation may be carried out in anumber of separate stages. Thus, there may be a leaching stage, ordissolution of zinc followed by a separating of the leaching residue,"and a reduction stage with a separating of the sulphurous residue. Theseparation of the "lead and sulphurous residues and the iron-baseprecipitate is done by any known processes such as filtration,decantation or by any other method. The lead residue may then be treatedby the conventional metallurgical methods for the recovering ofrecoverable insoluble elements such as lead, silver, tin and gold; asregards the sulphurous residue it may beforehand be treated to extractthe contained elementary sulphur and/or treated in roasting furnaceseventually mixed with sulphurous materials.

The method provided in the invention for oxidizing and thenprecipitating the iron is particularly advantageous because it makes itpossible to obtain a precipitate in the form of semi-hydrated ironhydroxides, such as a(Fe DOB) and 'y(Fe OOH), goethite andlepidocrocite, which are easy to filter or settle down. In this way areobviated all the difiiculties encountered when separating a ferrichydroxide precipitate Fe (Ol-l) obtained by the neutralization of aferric sulphate solution with a high pH value, while consuming less ofthe neutralizing agent, and also the loss of sulphuric acid and theaddition of an alkali in the case where, in accordance with certainknown methods, iron is being precipitated in the form of jarosite in amore acid medium, or else an expensive and complicated apparatus beingnecessary in the case where the iron is precipitated under high pressureand at high temperature. The dilute sulphuric acid solution used in thephase of treatment of the leaching residue for dissolving the zincpreferably consists of acid originating from electrolysis solutions. Forthe reduction raw zinc concentrate is advantageously used. has beenfound in the course of this operation that the sulphides of zinc andcopper are dissolved preferentially to sulphides of Pb and Fe, and thiswith high metallurgical yields. Through this operation it is possibletherefore to separate and extract from the mixed ores, the containedzinc and copper. Under the conditions of the invention, reduction yieldsof ferric iron in solution are obtained between and percent.

With regard to the oxidization phase, it should be treated that it isactivated by certain catalyzing elements, particularly solubilizedsilica and copper as well as active carbon. In the case of the treatmentof leaching residues enough copper is generally present to secure thiscatalytic action.

It should be noted that it is possible to carry out the treatment of thesolution after reduction in two stages, the first stage consisting of aneutralization of the free acid, the second being the precipitation ofthe iron itself.

This neutralization may be performed by roasted zinc concentrate or anyother neutralizing agent; the residue of this neutralization mayadvantageously be treated together with the leaching residue in thefirst stage of the present process.

Incidentally, the speeds of settling and filtration of the ironprecipitate have been measured. The following values were obtained:

Speed of settling: over 4 cm. per minute.

Speed of filtration: 500 kg. per sq. meter per hour of dry residue for acake of a thickness of 10 mm. and a pressure drop of 500 mm. mercury.

In one embodiment of the invention the said oxidization and the saidprecipitation are applied to a solution having a specific gravity whichmay be from 1.20 to 1.50; an iron content of 50 grammes per litermaximum, the Fe /Fe weight ratio in this solution being less than 0.05,the said solution containing more than 300 mg. per liter of copper ascatalyst and the grain size of the neutralizing agent being less than200 mesh.

The present invention will now be described in a more detailed butnon-restrictive manner in the following example which relates to aprocess carried out in three stages, i.e. a leaching stage or brininginto solution of the zinc, a reduction stage and a stage ofneutralization, oxidization and precipitation of the iron.

EXAMPLE lst stage 45 liters of pulp leaching residue containing 631grammes per litre of washed and dried solid products were mixed with 200liters of sulphuric acid originating from electrolysis cells (spentelectrolyte). The mixture was agitated and kept for 6 hours at atemperature of 90 to 85 C. The lead residue and the solution were thenseparated. The various elements in the loaded pulp on the one hand inthe lead" residue and solution obtained on the other hand were thendetermined. The results are shown in Table 1 below while in Table 2 thebalance of several elements is shown.

p charged Products obtained Filtrate Spent accompanyelectrolyte Leachinging the (liquid from Lead residue leaching electrolytic residue FinalElement (percent); residue cells) (percent) solution Total Zn 20.21 160gll 48 g/l 3.68 85.2 g/l I Zn soluble in H 0.19 0.32 Fe... 28.6 15.9526.7 g/l 8.43 19.74 0.12 330mgll 0.03 0.19g/1 1.26 380mg" 0.18 1.47gll0.15 0.4 0.0048 0.056 mg/l 0.01 mgll 0.0034 3.86 mgll 0.24 0.5 mgll0.125 mg/l 0.08 233.4 mg/l 0.06 0.6 mg/l 0.01 mgll 0.046 30 mg/l 320*720* 162 gll 30 g/l *g-per metric ton.

Weight of washed and dried lead residue 35 1 Weiglrtof washed and driedleaching residue The yield of dissolution of the Zn ie th a e 1 whlchhas een dissolved is equal to: ra of insoluble Insoluble Zn in theleachiiig'resfidue"'= Insoluble Zn in the lead residue Insoluble Znresidue of leaching residue 931% TABLE 2 Balance of several elements(9%) Pulp Fe Pb Ag Cu Cd As Sb Ge Filtrate accompanying 0 0 0 4.23 29.580.03 7.39 11.29 the residue Leaching 100 100 100 95.76 70.41 99.94 92.5988.70 residue Spent 0 l 0 0 0 0,03 0.01 0 electrolyte Products Finalsolution 74.03 0 0 97.54 87.10 82.37 48.19 81.70 Lead residue 24.04 10098.78 6.17 7.21 15.18 32.23 28.42 Loss or gain 1.91 0 1.22 3.72 5.692.44 19.57 -10.12

2nd stage 4,787 grammes of raw zinc concentrate were added to 175 litersof solution obtained from the first stage. Agitation was maintained for4 hours, the temperature varying between 95 and 100 C.

At the end of the operation 175 liters of solution were separated offand 2,202 grammes of sulphurous residue were collected. Analyses andproportions of certain elements are shown in Tables 3 and 4.

TABLE 3 Analysis of various products From these tables it is possible todeduce the yields of dissolution of the zinc and the yields ofdissolution of the iron contained in the raw concentrate:

Yield of dissolution of the zinc Yield of dissolution of the iron Thequantity of sulphur added as sulfide compared with the stoichiometricquantity necessary to reduce the ferris iron to ferrous iron, amounts to1.25.

The yield of reduction of ferric iron to ferrous iron 95.88

3rd stage 13,338 grammes of roasted zinc concentrate with a grain sizeless than 200 mesh were slowly added, over a period of 4 hours, to 175liters of solution coming from the second stage.

This operation took place at a temperature of C. and the pH value waskept between 1.7 and 3.

Then a quantity of separated equal to 5 times the stoichiometricquantity necessary to oxidize the iron, was blown in. The solution wasthen separated from the residue. Analyses of several elements containedin the solution and the roasted concentrate on the one hand and theproducts obtained on the other hand are given in Table 5, while Table 6gives the balance of several elements.

TABLE 5 Analysis of several products TABLE 6 Balance of several elements(percent) Zn Fe Cu As Sb Ge Cd F Feed:

Solution 64. 93. 19 95. 01 89. 08 81. 16 69. 85. 82 100. 0 Rst.concentrate-- 35. 52 6.81 4. 99 10. 92 18. 84 31. 00 14.1 Total .100. 00100. 00 100. 00 100. 00 100. 00 100. 00 100. 00 100. 0 Productsobtained:

Solution 100. 43 10. 63 84. 93 0. 01 3. 13 4. 67 100. 72 25. 2 Iron 89.08 22. 93 88. 94 115. 39 72. 02 2. 71 70. 7 Precipitate- +0. 29 7.8611.05 18. 52 23.3 3.43 4. 1 Loss or gain- 100. 00 100. 00 100. 00 100.00 100. 00 100. 00 1 00. 0

Zn of the neutralizing agent-Zn of the iron precipitate Zn of theneutralizing agent and on the other hand, the loss of copper due to thefact that part of the copper of the solution is precipitated:

Cu contained in the precipitate Cu contained in the neutralizing agentCu in the solution before iron precipitation This invention makes itpossible, among other things, to avoid the release of dangerous gasessuch as arsenamine, because the crude blende is used for the reductioninstead of metals. It has, furthermore, been found that when thereduction is carried out by means of a complex ore of zinc, lead, ironand copper, only zinc and copper sulfides are efiective as reducingagent. This fact may be of some interest when treating such complex oresin order to obtain a good recovery of zinc and copper.

In the course of the stage in which the iron is precipitated,

the elimination of certain impurities has been noticed, such as arsenic,antimony, germanium and fluorine, with better results than in otherknown processes.

The invention is of course not restricted to the example describedabove, and may be subject to modifications without going outside thescope of the invention defined by the following claims.

What I claim is:

1. In a process for the recovery of zinc values from zinc plant leachingresidue containing zinc sulphide and zinc ferrites, comprisingsubjecting the leaching residue to a subsequent leaching using a dilutesulphuric acid solution of a concentration of 100 to 250 grams ofsulphuric acid per liter, the sulphuric acid being added in a quantitytwo to five times the stoichiometric quantity required to dissolve thezinc contained in the leaching residue at a temperature between 50 C.and the boiling point of the leaching solution at atmospheric pressure,thereby to form a sludge of an insoluble lead residue containinginsoluble elements such as lead, silver and gold, and a solutioncontaining soluble elements such as zinc, copper, cadmium, ferric andferrous iron and unreacted sulphuric acid, separating the lead residuefrom the solution, reducing said ferric iron in said separated solutionto produce a sulphurous residue and a ferrous solution, and adding tosaid ferrous solution and neutralizing agent containing zinc oxide; theimprovement comprising conducting said reducing of ferric iron with asulphur bearing ore containing zinc sulphide in a quantity equal to 1.1to 1.5 times the quantity necessary for the reduction of ferric ironpresent in the said separated solution foat the atmospheric pressure,the temperature of the solution being between 50 C. and the boilingpoint of the solution, adding to said ferrous solution said neutralizingagent containing zinc oxide in a quantity to obtain and maintain a pHbetween 1.7 and 3, and oxidizing the said ferrous solution by passingair through the said ferrous solution at atmospheric pressure and atemperature between 50 C. and the boiling point of said solution,thereby precipitating iron in the ferric state along with othercollected impurities such as arsenic, antimony, germanium and fluorine.

2. A process as claimed in 1, wherein the reducing agent is a raw zincconcentrate.

3. A process as claimed in claim 1, wherein the reduction of iron insolution is carried out with a sulphur containing complex ore of zinccopper, lead and iron, of which the zinc and copper are dissolved theunreacted sulphuric acid.

A process as claimed In claim 1, wherein the neutralizing agentcontaining zinc oxide is added simultaneously with the air for oxidizingthe ferrous solution.

5. A process as claimed in claim 1, wherein the zinc plant residues aresimultaneously treated with dilute sulphuric acid and sulphur containingore, the resulting lead and sulphurous residue are separated from theferrous solution and thereafter the ferrous solution is simultaneouslyoxidized and neutralized thus precipitating the iron.

6. A process as claimed in claim 5, wherein the residue from theneutralization of the ferrous solution is combined with zinc plantleaching residues.

2. A process as claimed in claim 1, wherein the reducing agent is a rawzinc concentrate.
 3. A process as claimed in claim 1, wherein thereduction of iron in solution is carried out with a sulphur containingcomplex ore of zinc copper, lead and iron, of which the zinc and copperare dissolved in the unreacted sulphuric acid.
 4. A process as claimedin claim 1, wherein the neutralizing agent containing zinc oxide isadded simultaneously with the air for oxidizing the ferrous solution. 5.A process as claimed in claim 1, wherein the zinc plant residues aresimultaneously treated with dilute sulphuric acid and sulphur containingore, the resulting lead and sulphurous residue are separated from theferrous solution and thereafter the ferrous solution is simultaneouslyoxidized and neutralized tHus precipitating the iron.
 6. A process asclaimed in claim 5, wherein the residue from the neutralization of theferrous solution is combined with zinc plant leaching residues.